This discussion appears about over, but I haven't seen any solutions via inheritance. Other languages that lack true interfaces use abstract base classes. Python supports multiple inheritance. Isn't this enough? If the basic types are turned into abstract base classes and inserted into the builtin name space, and library and user-defined classes are reparented to the appropriate base class, then isinstance becomes the test for category inclusion. A partial example: class FileType(): def __init__(*args, **kwargs): raise AbstractClassError, \ "You cannot instantiate abstract classes" def readline(*args, **kwargs): raise NotImplementedError, \ "Methods must be overridden by their children" All "file-like" objects, beginning with file itself and StringIO, can extend FileType (or AbstractFile or File or whatever). A function expecting a file-like object or a filename can test the parameter to see if it is an instance of FileType rather than seeing if it has a readline method. Type hierarchies could be obvious (or endlessly debated): Object --> Collection --> Sequence --> List \ \ \--> Tuple \ \ \--> String \ \--> Set \ \--> Mapping --> Dict \--> FileLike --> File \--> StringIO \--> Number --> Complex \--> Real --> Integer --> Long \--> Float \--> Iterator \--> Iterable etc. The hierarchy could be further complicated with mutability (MutableSequence (e.g. list), ImmutableSequence (e.g. tuple, string)), or perhaps mutability could be a property of classes or even objects (allowing runtime marking of objects read-only? by contract? enforced?). This seems to be a library (not language) solution to the problem posed. Can the low level types implemented completely in C still descend from a python parent class without any performance hit? Can someone please point out the inferiority or infeasibility of this method? Or is it just "ugly"? -- Nathan Clegg GeerBox nathan@geerbox.com